The present invention relates to a method of producing elastic foam having a base of polyurethane particularly useful for damping sound in automobiles by foaming dielectric heating.
In addition, the invention concerns an elastic foam as well as a sound-absorbing composite structural part based on this foam.
The sound-damping and acoustic insulating of the engine compartment of motor vehicles, etc., is acquiring greater and greater importance since noise pollution in the environment and noise directed at the passengers in a motor vehicle should be reduced as much as possible. Sound-damping has up to now been effected technically predominantly by means of hot-shaped parts of soft-foam which are fastened by bonding or mechanical fastening techniques to the body of the car or to supporting plastic parts installed therein. Foams for the damping of sound in the engine compartment should have good acoustic sound-damping action, good mechanical strength, sufficient elasticity and good restoration behavior, low bulk weight, difficult inflammability, chemical resistance to water, oil and gasoline, as well as resistance to temperatures of up to 180.degree. C. It is difficult to satisfy these conditions simultaneously.
Furthermore, the manufacture of the foam parts is effected in numerous individual steps which are expensive in terms of labor, energy and time. In particular, pre-cutting or stamping of the foam parts from a larger block and bonding or mechanical attachment to supporting plastic parts must be effected.
It is therefore endeavored to manufacture the foam parts directly in the desired shape in as few operations as possible. The foaming and hardening process is, however, difficult to carry out in the case of such small foam parts.
The best method for this is foaming under dielectric heating, for instance by microwaves, as described in Federal Republic of Germany patent document No. OS 30 11 769. The monomer mixtures used up to now for the purpose intended here are, however, not suitable for foaming under dielectric heating.